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The oceans mediate the response of global climate to natural and anthropogenic radiative forcing, yet observations of global maritime surface climate variations in the late Holocene, and the mechanisms that drive the variations, are relatively unknown. Here we synthesize 57 sea surface temperature (SST) reconstructions, sourced from all major ocean basins, and spanning at high resolution some or all of the past 2000 years. The reconstructions are derived from marine archives (Mg/Ca, alkenones, TEX86, faunal assemblages in sediment cores, and coral), and meet strict chronological control criteria. The reconstructions are geographically sparse, however analysis of multi-millennial AOGCM output and historical gridded SST observations suggest the reconstructions are spatially sufficient to resolve global mean SST.

The reconstructions were standardised into 200-year bins and the resulting Ocean2k SST synthesis reveals a robust SST cooling trend for 0-1800 years of the Common Era (CE), with the strongest cooling after 1100 CE. The cooling trend is not sensitive to localized upwelling, marine archive type, seasonality of response, chronological control, water depth, sampling resolution, sedimentation rate, basin, latitude or hemisphere.

The Ocean2k SST cooling trend is qualitatively consistent with an independent synthesis of terrestrial paleoclimate data, and with simulations from the multimodel PMIP3 ensemble, driven by the full suite of hypothesized radiative forcings. Comparison with ensembles of single and cumulative radiative forcing simulations suggests that the cooling trend arises not from orbital forcing, but from the increased frequency of explosive volcanism and/or land use change in the most recent millennium. We find that episodic volcanic eruptions induce a net negative radiative forcing that results in a centennial and global-scale cooling trend via a decline in mixed-layer oceanic heat content.

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Volcanic eruptions are known to be a major driver of climate variability during the last millennium (the Common Era). Recent high-resolution polar ice core data reveal new insights into volcanic forcing during this period, improving our knowledge of the timing and the amount of sulfur released by past volcanic eruptions (Sigl et al., 2014). Here we focus on the effect of volcanic “double events”, such as the major eruptions of 1808/1809 (Unknown) and 1815 (Tambora). The close temporal proximity of two eruptions creates the potential for climate impacts to superimpose. Ice core records provide evidence of other such double events, and climate proxy records are used to assess the climate impact of double compared to single eruption events. We perform climate model simulations to investigate specific volcanic double events, reconstructed from ice core records, and assess the potential for additive climate impacts. In particular, simulations are used to investigate the impact of decadal scale volcanic radiative and dynamical anomalies on the coupled climate system including vegetation, sea ice and ocean circulation changes. Our model results show that given certain conditions, two closely spaced eruptions of Tambora magnitude could have a larger cumulative climate impact than a single eruption of much greater magnitude. Based on the observational records and model results, we propose that volcanic double events are a likely agent for abrupt (decadal) climate changes, and may have had significant impacts on past civilizations.

Climate change making impacts on societies in the European/Mediterranean region during the past two millennia as seen by land-ocean palæoarchives

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A composite of sea surface temperature (SST) reconstructions from the Mediterranean/Eastern Atlantic basins (this study; McGregor et al., 2015, submitted) is compared with a stack of terrestrial records selected to describe the temperature variability in the European region over the past 2000 years (PAGES 2k consortium, 2013, NATURE GEO.). Prior to 800 CE, the scarcity of measurements prevents us from making sound inferences about climatic changes during this interval. From 800 to 1800 CE, both land and ocean signals present common significant cooling trends (up to –1 s. d. units/millennium, R2>0.67, p<0.01), attributable to the anomalous cold temperatures registered during the Little Ice Age. The latter is a well-documented anomaly, which had strong impacts on European societies and stands out in both composites, particularly after 1600 CE. Ensembles of forcing simulations suggest that this is consistent with a global cooling trend, which arises from increased frequency of explosive volcanism and possible land use change. During the post-industrial time interval, the terrestrial composite registers almost +2 s. d. units warming since 1800 CE, in remarkable agreement (R2=0.81, p=0.01) with a reference Central England temperature record estimated independently (Met Office Hadley Centre for Climate Change, 2014). The warming is of lower amplitude in the ocean composite, only +1 s. d. units, in line with historical sea surface temperatures (Kaplan et al., 1998, J. GEOPHYS. RES.). The 20th century warming amplitude seems more pronounced as more low-latitude and close-to-land records are taken into consideration, whereas inclusion of high-latitude and/or upwelling locations attenuates it. Given the uncertainties inherent in the reconstructions available and the variety of responses to climate changes during this interval, the answer to the question whether the last century is likely the warmest climatic period of the latest 2000 years appears elusive in the Europe/Mediterranean region.

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Early documentary records report of a mysterious dust cloud that was covering Europe for 12 months in 536-37 CE, which was followed by climatic downturn and societal decline globally. Tree rings and other climate proxies have corroborated the occurrence of this event as well as characterized its extent and duration, but failed to trace its origin.

By using a multi-disciplinary approach that integrates novel, global-scale time markers with state-of-the-art continuous ice core aerosol measurements, automated objective ice-core layer counting, tephra analyses, and detailed examination of historical archives, we developed a new volcanic forcing series from bipolar ice-core arrays back into Roman times. We revised the timing of major volcanic eruptions and reconstructed atmospheric aerosol loading and the spatio-temporal distribution of volcanic sulfate and tephra. Precise ice-core timescales for Greenland and Antarctica enabled us to discern tropical from Northern Hemisphere eruptions, and to identify the climate-altering role of non-tropical eruptions, such as those causing the “mystery cloud” in 536-37 CE.

Our study reconciles human and natural archives – demonstrated by the synchronicity of major volcanic eruption dates to historical documentary records and the (now) consistent response of tree-ring-reconstructed cooling extremes occurring in the immediate aftermath of large volcanic eruptions throughout the past 2,000 years.

These findings have significant implications in multiple research fields including quantification and attribution of climate variations to external forcing, and will be of benefit for historians, volcanologists as well as climate scientists.

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The 1259 AD unknown volcanic eruption found in polar ice cores is considered as the most powerful eruption of the last 7 000 years and could have triggered the little ice age as well as profoundly changed the political and sociological organization of the middle age. Despite its relative young age and power, the origin of this eruption has remained elusive. For years, volcanologists and glaciologists have searched for the possible origin of this mega eruption with little results. This event has thus remained as the unknown 1259AD event, limiting a possible examination in great details of the climatic consequences of this eruption. After decades of research, based on datation, geomorphological considerations and historical archives, Lavigne et al [1] have recently proposed the Salamas/Rinjani (Indonesia) as the possible volcano for this eruption.

In order to validate this proposition, we have undertaken a thorough and meticulous analysis of the geochemical composition of the tiny volcanic glasses found in the ice layer corresponding of this eruption. With a great level of certainty, the chemical fingerprint of the tiny glasses in ice is found to match the chemical compositions of the ashes layers found around the volcano. This result should open a complete review of the climatic impact of this volcano and demonstrated the feasibility of identifying the origin of volcanoes in ice cores from their geochemical fingerprint.

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Ice cores provide an opportunity to reconstruct a history of Earth’s volcanic activity. Many attempts have been made in the past to build a robust volcanic climate forcing model based on the ice records, but the robustness of this model depends on the statistical significance of usually a single record and on discriminating between large distant eruptions and closer, smaller eruptions.

We used ice cores drilled at Dome C (Antarctica) to measure first, the statistical weight of a single ice core record and second, to measure the non-zero Δ33S of ice-core sulfate as a proxy for stratospheric volcanic eruptions. SO2 emitted by these events can be mass-independently fractionated (Δ33S≠0) during photolysis reactions in the stratosphere [1], [2], [3], [4]). The signal, recorded in aerosol sulfate, is preserved in ice cores. The sulfur anomaly allows identification of eruptions very likely to be stratospheric, with potential climatic impact, regardless to the ice core location, the age or the magnitude of the recorded event.

In 2010-2011, five 100m-long ice cores from Dome C, Antarctica, separated by 1m, were collected to reconstruct the history of volcanism over the last 2500 years. Assumed volcanic events were identified through sulfate concentration measurements in the field, and located on the cores. We used an algorithm for the peak detection, and 51 potential volcanic events were identified. Based on this detection, a statistical evaluation of the occurrence of these 51 events in each ice core was conducted and allowed measuring the representativeness of a single ice core to reveal a history of volcanism. Following this statistical work, volcanic sulfate contained in snow and ice have been isolated, decontaminated, melted, concentrated and extracted using ion exchange methods. Each presumed volcanic event has been subdivided in 5 fractions at least, in order to differentiate the background isotopic signal from the sulfate peak. The peak itself has been divided into three portions. The results show that Δ33S and Δ36S are anti-correlated, and allow to discriminate stratospheric and tropospheric events, with the former having isotopic systematics that vary first with a positive Δ33S at the beginning of deposition and with a negative Δ33S at the end of the volcanic deposition. Such unique feature should allow us to reconstruct a robust volcanic forcing signal, independent of the size of the event record in ice.

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It is now generally recognised that volcanic eruptions have an important effect on climate variability from inter-annual to decadal timescales. At the decadal scale, CMIP5/PMIP3 simulations have highlighted that clusters of eruptionscan produce a long-lasting cooling such as during the 13th and the 19th centuries. This has led to the hypothesis that enhanced volcanic activity during the second half of the 13th century played a significant role in the transition towards the Little Ice Age through its impact on the thermohaline oceanic circulation and on Arctic sea ice. This hypothesis remains however speculative, as paleoclimatic reconstructions and climate simulations yield partly contradictory results.

Concerning the short-term hemispheric response to sulphur-rich volcanic eruptions, CMIP5/PMIP3 climate simulations show stronger cooling than tree-ring based reconstructions. Scarce information about the medieval and 19th century eruptions (i.e. magnitude, timing and location) have so far hampered a realistic assessment of the climatic impacts of decadally-paced volcanic events. Another possible reason for the mismatch is our limited understanding of the evolution of stratospheric aerosol formed after strong eruptions of pre-instrumental period, such as Tambora in April 1915, for which no direct observations are available.

It is therefore necessary to frame future modeling activities within common designs that separately tackle the two major steps linking sulfur emissions by volcanic eruptions and climate response: first, the chemical and microphysical transformation occurring within the stratospheric volcanic cloud; then, the aerosols’ direct radiative effects and associated feedback mechanisms activated in the coupled ocean-atmosphere system. The coordinated modeling initiative within VOLMIP (Model Intercomparison Project on the climatic response to Volcanic forcing, an activity forCMIP6-) has been motivated by such necessity. VolMIP will provide new consensus forcing input data and related coupled climate simulations for some of the major volcanic eruptions that occurred during the pre-industrial period of the last millennium.

This contribution presents ongoing activities and research highlights achieved within VolMIP, illustrating how these coordinated modeling assessments are contributing towards constraining uncertainties in the climate response to volcanic forcing, improving the evaluation of climate models, and advancing our understanding of past, current and future climates.